The concept of biocompatibility of the materials used in surgical reconstruction of joints, ligaments, and tendons is controversial, as evinced by the conflicting definitions proposed by the many authors who have studied the host reaction to the presence of implants and their breakdown products. We propose that biocompatibility of contemporary medical implants is not a property of the chemical composition of the biomaterials but depends rather on their physical attributes. The histological reaction patterns of tissue to the presence of polyethylene in diverse physical states are described. The inflammatory response evoked by the implants is laudable in so far as it precedes and accompanies the adequate tissular incorporation of the devices used. On the other hand, the granulomatous reaction induced by small, irregularly shaped and edgy breakdown products adversely affects the life span of the implants. Thus the manner in which the host handles the biomaterials is determined primarily by the physical state of the biomaterials (rather than their chemical composition), which in turn determines the success or failure of reconstructive surgery. It logically follows that biocompatibility constitutes a relativistic concept.